1. Field of the Invention
The present invention relates to a flexible wiring board and particularly to a characteristic impedance control circuit for a high-frequency signal.
2. Related Art
A flexible wiring board 10 has been incorporated in a small-sized electronic device as shown in FIG. 7 in many cases, and particularly a cable portion is folded along a folding line shown by a broken line and incorporated in equipment. In this case, an impedance control line 20 is sharply folded at a portion of the folding line and the characteristic impedance is rapidly changed at this folded portion.
That is, in the characteristic impedance control circuit of a high-frequency circuit formed on a flexible wiring board, electromagnetic-field distribution is distorted at a folded portion in the circuit (“microstrip line” in Patent Document 1), which might adversely affect circuit characteristics such as reflection and irradiation or malfunction of other circuits as described in [Background Art] in Patent Document 1.
As a measure against that, it is possible to form the folded portion in a characteristic impedance control circuit as long as it is on a plane (Patent Document 1).
However, when three-dimensional folding is to be performed at an assembling stage of the flexible wiring board into electronic equipments, the change will be made at a steep angle in the state where the characteristic impedance control circuit is folded as in Patent Document 1.
As a result, the electromagnetic-field distribution is distorted and signal reflection or irradiation is generated, which might cause adverse effect of the circuit characteristic or malfunction of other circuits.
Patent Document 1: Japanese Patent Laid-Open No. 2004-363535